Semiconductor-type solid state switches are used extensively in various industrial applications today. Such solid-state elements have the advantages such as no mechanical contact, therefore no sparkling and fast triggering, and when such elements are used for switching on/off the high-powered system, it is very safe and durable. Now a day, they are indispensable for modern industrial electrical power control. Among these, the solid-state switch devices which are used most extensively are silicon-controlled recifier (SCR) and TRIAC that is obtained by connecting two SCRs in anti-parallel status.
Using a SCR for triggering large current has the following disadvantages: (1) the transimpedance between the input and output terminals of a SCR is small, therefore the two terminals are not isolated from each other; if a high voltage appears at the output end, it is likely to have a chance causing breakdown at the input circuit; (2) any SCR element has the characteristics of latch-up after triggering, once a SCR is triggered to be in conducting state, it will not be terminated until the potential difference between anode and cathode is zeroed, or unless an inverse current is injected into the gate. These are different from the traditional mechanical switch having high-transimpedance characteristic, i.e. the electrical isolation between the input and output ends is complete, and latch-up property is naturally absent. From the above mentioned, there are intrinsic disadvantages for the use of an SCR.
To achieve the functions of a traditional mechanical switche without losing the advantages of non-contact and no sparkling, there is another new type of solid-state switching device used increasingly in recent years. The electricity-driving of the device is achieved by high voltage, high power enhancement-mode MOS transistors which are normally off. Since a MOS transistor is a bilateral device with very high input impedance to its input gate, it behaves similarly to a conventional mechanical switche. When the device is to be driven to turn on, we need only apply a threshold voltage to the gate. For a power MOS transistor the threshold voltage is approximately 3 volt, and since the MOS transistor has a very large input impedance, the required current and power for driving this element are both small. The MOS transistor can be driven to turn on by charging the gate capacitor thereof, whose capacitance is very small; when the external driving voltage is removed, the gate stray charge decreases gradually due to through certain leakage loop and until its voltage drops down below the threshold voltage then the MOS transistor becomes off. Besides, since the MOS transistor is a bilateral element, it can be used for driving the alternating power under appropriate design. In this way, it is like the mechanical switch.
To reach a triggered conduction of the power MOS transistor, there must be a circuit generating a sufficient potential over the threshold value, which is applied between the gate and source. Furthermore, there must be very high electrical isolation between the input terminal and the output terminal such that the above-mentioned disadvantages of the SCR can be avoided. At present, it is common to utilize "photovoltaic effect" of the diode photo cell, as shown in FIG. 1. In FIG. 1, there are a plurality of silicon photodiodes in series connection before the gate G of the power MOS transistor Q, the series-connected diodes are illuminated by a light emitting diode LED, a photo-voltage is thus generated. In general, the photo-voltage generated by a silicon diode is approximately 0.3.about.0.6V, so the structure of ten series-connected silicon diodes can generate a voltage higher than the threshold voltage (about 3V) of the power MOS transistor, which can turn on the power MOS transistor. The "photovoltaic generator" (PVG) switch of such photoelectric coupling type is similar to those of the photo coupler or opto isolator used in communication electronics. Since there is no circuit connection between the light emitting diode and silicon diodes, the PVG switch has very large transimpedance. This type of photovoltaic generator switch can drive an output load having very high voltage and current at the output end of the MOS transistor using a small lighting power at light emitting diode end, which does not result in electrical breakdown at the input. It is a new and excellent solid-state power electronic switch at present, which can be referred to in product catalogs of many corporations.
However, when this type of MOS power switch driven by the "photovoltaic generator" is at the beginning of turning off, i.e. when the light of the LED is turned off, there is parasitic charge stored at the gate capacitor C.sub.G (.about.120 pf) of the power MOS, which must be leaked away so that the power MOS can be turned off. However, since then the diode is under high impedance of negative bias condition, the charge is not easy to leak away. Therefore, the MOS transistor can not responds and becomes off instantaneously. As an electricity switch, such delay at the turning off is very undesirable, if it is too long to harm the switching operation. To overcome the disadvantage, a circuit which provides discharging path is indispensable between the gate and the output of the photocell for the present "photovoltaic generator MOS switch" (PVG switch), as illustrated by the block S in FIG. 2. The existence of such circuit results in not only an increase of total cost due to chip area being increased and process being more complex, but also an increase demand of total photovoltaic power to be generated, which means the number of light emitting diodes has to be increased or the chip area of the photocells has to be enlarged to replenish, this again leads to the increase in cost. Furthermore, it becomes more difficult to make the light of the LED uniformly irradiating on the surfaces of the photo cells.
Besides, as shown in FIG. 3A, concerning the package structure of the photovoltaic generator solid-state switch, since the light emitting diode and the photo cell have to face to each other such that direct optical coupling is possible, the package structure is more complicated than the planar package structure of ordinary IC, as shown in FIG. 3B, the cost of package is thus considerably higher. Besides, the package epoxy between the two elements must be transparent for the light of the LED wavelength, otherwise photo coupling is hardly performed. For the present epoxy material used for IC package, the light source used must be an infrared LED which emits light with wavelength around 900 nm able to transmit through the epoxy material for the coupling. The silicon photo cell, which is quite cheap, has higher photoelectric transformation efficiency in the spectrum section.
Using single-chip silicon photo cell has the following disadvantages. Since the silicon photodiodes used by the photovoltaic generator switch have to be connected in series for generating sufficient photovoltaic voltage to drive the power MOS transistor, there is difficulty when the elements are formed on one silicon chip, as explained in the following: (1) There must be very high electrical isolation between adjacent diodes such that series connection takes effect, otherwise leakage will be brought about and induced voltage and current decrease; (2) The absorption length of single crystal silicon for the wavelength of 900 nm is quite large, therefore the photodiodes on the silicon chip must have deep junctions such that photoelectric conversion efficiency can be raised to a reasonable level, which leads to even deeper isolation between adjacent elements. This deep isolation is hard to implement on ordinary silicon chips. To achieve the object, silicon-on-insulator (SOI) waver is utilized in many present products such that isolation between adjacent elements can be accomplished, as the structure shown in FIG. 4. However, the SOI wafer is more expensive as compared with ordinary single crystal silicon wafer and the manufacturing process of the SOI chip is also more complicated. In addition, the thickness of silicon on the isolation layer cannot be increased too much, otherwise V-groove will be too deep to be made flat, which leads to the decrease of photo absorption efficiency.
In short, although the power MOS solid-state switch device of photovoltaic generator has been put into practice at present, however, as described in the above, the disadvantages related with the material, manufacture, package structure of the elements altogether make such solid-state switch cost high or have unnegligible disadvantages.